In production step of a semiconductor integrated circuit device, circuit elements and/or wirings are formed on a semiconductor wafer by forming a resist pattern on the semiconductor wafer, etching a thin film (e.g., an oxidized film or a deposited film) on the semiconductor wafer using the resist pattern as a mask, and injecting impurity ions into specific regions of a substrate.
For forming a resist pattern on a semiconductor wafer, a photoresist is first spin-coated on the semiconductor wafer. The photoresist used is a composition prepared by diluting, for example, a novolak resin and a photosensitizer with an organic solvent. After removing the organic solvent by heating the photoresist thus formed, latent images are formed at the desired pattern regions by irradiating the surface of the photoresist with an exposing light such as an ultraviolet ray, an electron beam, laser, etc. The photoresist is then developed by, for example, a wet treatment with an alkali developer.
On the other hand, the resist pattern which has become unnecessary after etching step or the ion-injection step is removed from the semiconductor wafer by ashing. As the ashing method, there are a method cf using O.sub.2 plasma and a method of using ultraviolet rays and ozone. In addition, the ashing technique is described in, e.g., '90 Saishin Handotai Process Gilutsu (Newest Semiconductor Process Technique), page 220-229, published by K.K. Press Journal, Nov. 2, 1992.
However, the conventional ashing technique involves a problem that impurities such as sodium, heavy metals, etc., contained in a photoresist remain on the semiconductor wafer after ashing and these impurities diffuse in the inside of the wafer by the subsequent heat-treatment to deteriorate the characteristics of the semiconductor element.
Furthermore, the conventional ashing technique also has a problem that it is difficult to remove the resist pattern denatured by the ion injection.